This invention relates to laser electrophotographic printers and more specifically relates to an image scanning apparatus providing improved imaging in both the scan and process direction using a multiple spot printhead.
Electrophotographic printers are designed to create an image by placing a series of picture elements (pels) on an image receiving material. For example, an image may be created by a light source, such as one or more lasers, which is caused to scan across a photoconductor surface in a succession of scan lines. The light beam or beams place a series of overlapping pels on the photoconductor surface. Each pel is placed in a pel area and the light beam is modulated so that some pel areas are exposed to light and some are not. Whenever a pel containing light strikes the photoconductor surface, the surface is discharged at that pel location. In this manner, the photoconductor surface is caused to bear a charge pattern of pels which matches the object image that is to be reproduced. The printed copy is obtained by developing the charge pattern and transferring the developed image to print material, usually paper.
Electrophotographic printers are well known in the art and are described, for instance, in U.S. Pat. No. 4,544,264 issued to Larry W. Bassetti, dated Oct. 1, 1985 and U.S. Pat. No. 4,625,222, issued to Larry W. Bassetti et al, dated Nov. 25, 1986, both assigned to the assignee of the present invention.
The resolution of images produced by a laser electrophotographic printer is generally stated in terms of pels per inch. A typical printer, commercially available today, has a resolution of 480 pels per inch.
Increased performance requirements of electrophotographic printing has placed a heavy burden on the printheads. Increased speed, pel density and the like require higher rotating polygon mirror speeds, more facets on the polygon mirror and reduced pel times. Higher rotating polygon mirror speeds, in turn, result in larger motor mechanisms and more complex synchronization mechanisms.
Multiple spot imaging offers an opportunity to provide the sought after improved performance while overcoming the concerns cited above. By scanning multiple spots or pels in lieu of a single scan line, the printing speed is increased. Preferably two to six laser beams are used in order to achieve a favorable balance between speed, pel density and laser power versus polygon spinner considerations. For example, in the IBM 3800 model III printer, an acousto-optic modulator is used to split a HeNe laser into two, independently controlled, laser beams.
In addition, many systems have been proposed to optically combine semiconductor laser outputs to perform multiple spot imaging. Several problems, some of a fundamental nature, have prevented such systems from emerging as viable technologies.
An objective of multiple spot printers is to provide an image which has a spacing equal to one pel in the process direction and uniform pel displacement in the scan direction. In the present invention, the result is achieved in two steps. First, an image is generated which has the required one pel spacing in the process direction and no separation in the scan direction. Subsequently, an image is generated having uniform pel displacement in the scan direction.
The term "scan direction" refers to the direction of scan of the light beam across the photoconductor surface. The term "process direction" refers to the direction of movement of the photoconductor surface upon which the image is projected. In simple terms, the scan direction may be thought of as left-to-right and the process direction as top-to-bottom. Of course, the directions may be reversed or interchanged as determined by the light scan system and photoconductor motive system.